From b1d5552838334c600b068c9c8cc18638e5a8cb47 Mon Sep 17 00:00:00 2001 From: Mikulas Patocka Date: Thu, 12 Aug 2010 04:14:02 +0100 Subject: [PATCH] dm snapshot: implement merge Implement merge method for the snapshot origin to improve read performance. Without merge method, dm asks the upper layers to submit smallest possible bios --- one page. Submitting such small bios impacts performance negatively when reading or writing the origin device. Without this patch, CPU consumption when reading the origin on lvm on md-raid0 was 6 to 12%, with this patch, it drops to 1 to 4%. Note: in my testing, it actually degraded performance in some settings, I traced it to Maxtor disks having problems with > 512-sector requests. Reducing the number of sectors to /sys/block/sd*/queue/max_sectors_kb to 256 fixed the read performance. I think we don't have to care about weird disks that actually degrade performance because of large requests being sent to them. Signed-off-by: Mikulas Patocka Signed-off-by: Alasdair G Kergon --- drivers/md/dm-snap.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c index a1f2ab553b92..96feada5e761 100644 --- a/drivers/md/dm-snap.c +++ b/drivers/md/dm-snap.c @@ -2171,6 +2171,21 @@ static int origin_status(struct dm_target *ti, status_type_t type, char *result, return 0; } +static int origin_merge(struct dm_target *ti, struct bvec_merge_data *bvm, + struct bio_vec *biovec, int max_size) +{ + struct dm_dev *dev = ti->private; + struct request_queue *q = bdev_get_queue(dev->bdev); + + if (!q->merge_bvec_fn) + return max_size; + + bvm->bi_bdev = dev->bdev; + bvm->bi_sector = bvm->bi_sector; + + return min(max_size, q->merge_bvec_fn(q, bvm, biovec)); +} + static int origin_iterate_devices(struct dm_target *ti, iterate_devices_callout_fn fn, void *data) { @@ -2188,6 +2203,7 @@ static struct target_type origin_target = { .map = origin_map, .resume = origin_resume, .status = origin_status, + .merge = origin_merge, .iterate_devices = origin_iterate_devices, }; -- 2.20.1